The present invention relates to a magnetic field generator used in a medical-use magnetic resonance imaging (hereinafter referred to as MRI) device, and more particularly to an MRI magnetic field generator with reduced residual magnetism and eddy current generated by the effect of the pulse current flowing through Gradient magnetic field coils.
FIGS. 13(a) and (b) illustrate a known structure of an MRI magnetic field generator. In this structure, each of a pair of pole pieces 2 is fastened, with the pole pieces 2 facing each other, at one end of each of a pair of permanent magnet structures 1 comprising a plurality of block-shaped R-Fe-B-based magnets that have been integrated as the field generation source the other ends of the permanent magnet structures are connected to a yoke 3, and a static magnetic field is generated within the air gap 4 between the pole pieces 2.
In the figure, 5 is an annular protrusion formed in order to increase the uniformity of magnetic field distribution within the air gap 4, and another known structure is one in which a tiered protrusion (not shown) is formed on the inside of the annular protrusion in an effort to further increase the uniformity of the field distribution.
In the figure, 6 is a tilt field coil, which is disposed in order to obtain information about positioning within the air gap 4. These Gradient magnetic field coils 6 usually comprise a group of three coils corresponding to the three directions X, Y, and Z within the air gap 4, but are shown in simplified form in the figure.
With a structure such as this, the air gap 4 must be large enough for all or part of a patient""s body to be inserted therein, and a static magnetic field having a high uniformity of 1xc3x9710xe2x88x924 or less at 0.02 to 2.0 T must be formed within a specified image field of view within the air gap 4.
With the structure shown in FIGS. 13(a) and (b), a so-called four-column yoke consisting of a pair of yoke plates 3a and 3b and four yoke columns 3c is used as the yoke 3, but as shown in FIGS. 14(a) and (b), variously structured yokes can be used according to the required characteristics, such as a so-called C yoke consisting of a pair of yoke plates 3a and 3b and a supporting yoke plate 3d. 
With the structure shown in FIGS. 13(a) and (b), permanent magnets such as R-Fe-B-based magnets are employed as the field generation source, but other structures can also be used, such as one in which an electromagnetic coil is wound around the periphery of an iron core.
Regardless of which of these structures is used, the air gap 4 is formed by the pair of pole pieces 2, and Gradient magnetic field coils 6 are disposed in the vicinity of the pole pieces 2, as shown in FIGS. 13(a) and (b).
Usually, the pole pieces 2 are made from electromagnetic soft iron, pure iron, or another such bulk material (integrated), so when a pulse current is passed through the Gradient magnetic field coils 6 and a pulse-form tilt field is generated in the desired direction in order to obtain information about positioning within the air gap 4, the effect of this tilt field generates an eddy current in the pole pieces 2 and decreases the rise characteristics of the tilt field, and even after the flow of the pulse current has been halted, the uniformity of the field distribution in the air gap 4 is decreased by the residual magnetism generated in the pole pieces 2.
As a means for solving this problem, MRI magnetic field generators characterized in that the main portion of the pole pieces is formed from laminated silicon steel sheets have already been proposed by the inventors (Japanese Patent No. 2,649,436, Japanese Patent No. 2,649,437, U.S. Pat. No. 5,283,544, and European Patent No. 0479514).
The MRI magnetic field generators previously proposed by the inventors are chiefly characterized by the use of pole pieces structured as shown in FIGS. 16 to 18.
The structure of the pole piece 10 shown in FIGS. 15(a) and (b) comprises a soft iron magnetic ring having a rectangular cross-sectional shape and constituting an annular protrusion 12 on the air gap-facing side of a magnetic base member 11 composed of pure iron or another bulk material, and a plurality of laminated blocks 13 produced by laminating a plurality of silicon steel sheets in the facing direction of the pole pieces and integrating these with an insulating adhesive agent or the like.
In the figure, 14 is a tiered protrusion formed on the inside of the annular protrusion 12 for the purpose of enhancing the uniformity of the field distribution. Just as discussed above, a plurality of silicon steel sheets are laminated in the facing direction of the pole pieces and integrated with an insulating adhesive agent or the like, and the resulting plurality of laminated blocks are laminated in the required number.
15 in the figure is a soft iron core used for mounting the field generation coil.
16 in the figure is a slit formed in the radial direction for the purpose of dividing the soft iron magnetic ring having a rectangular cross-sectional shape and constituting the annular protrusion 12 into a plurality of sections in the circumferential direction and reducing the eddy current that is generated at the annular protrusion 12.
If the silicon steel sheets used in the above-mentioned laminated blocks 13 are directional silicon steel sheets (JIS C 2553, etc.), then from the standpoint of field distribution uniformity, it is preferable for them to be laminated and integrated such that the readily magnetizable axis direction (calendering direction) is rotated by 90 degrees every specific number of small blocks 13a and 13b as shown in FIG. 16(a). If the sheets are non-directional silicon steel sheets (JIS C 2552, etc.), then lamination and integration are performed merely in the thickness direction, without taking directionality into account, as shown in FIG. 16(b).
The structure of the pole pieces 20 shown in FIGS. 17(a) and (b) comprises a soft iron magnetic ring having a rectangular cross-sectional shape and constituting an annular protrusion 22 on the void-facing side of a magnetic base member 21 composed of pure iron or another bulk material, and a plurality of laminated blocks 23 produced by laminating a plurality of non-directional silicon steel sheets in the direction perpendicular to the facing direction of the pole pieces and integrating these with an insulating adhesive agent or the like.
In the figure, 24 is a tiered protrusion formed on the inside of the annular protrusion 22 for the purpose of enhancing the uniformity of the field distribution, 25 is a soft iron core used for mounting the field generation coil, and 26 is a slit that divides the soft iron magnetic ring having a rectangular cross-sectional shape and constituting the annular protrusion 22 into a plurality of""sections in the circumferential direction.
It is preferable for the above-mentioned laminated blocks 23 to be laminated and integrated with an insulating adhesive agent or the like such that the lamination direction is rotated by 90 degrees for every one of the small blocks 23a and 23b laminated in the void-facing direction, as shown in FIG. 17(c).
The structure of the pole pieces 30 shown in FIGS. 18(a) and (b) is quite different from that of the pole pieces 10 and 20 shown in FIGS. 15 (a) and (b) and FIGS. 17(a) and (b), respectively, in that the magnetic base members 11 and 21 composed of a bulk material are not used. Specifically, this structure is such that, instead of the magnetic base members 11 and 21 composed of a bulk material, laminated rods 33, produced by laminating a plurality of non-directional silicon steel sheets, as shown in FIG. 18(c), in the direction perpendicular to the facing direction of the pole pieces and laminating these with an insulating adhesive agent or the like, are supported by an annular support member 34 composed of a bulk magnetic material.
To discuss this in more detail, the center portion of the annular support member 34 composed of a bulk magnetic material is cut out, and the above-mentioned laminated rods 33a are disposed unidirectionally suspended therein with the chamfers 38 thereof corresponding to the chamfers (not shown) formed around the inside edges of the cutout. The laminated rods 33b are laid out as a second layer such that the lamination direction is rotated by 90 degrees on the void-facing side of the laminated rods 33a. 
A plurality of laminated rods 33c of different length are disposed between a fixed plate 35 and the inner peripheral surface of the annular support member 34 so that the overall shape of the pole piece will approximate that of a disk, and a soft iron magnetic ring having a trapezoidal cross section and constituting the annular protrusion 32 is installed via fixed blocks 31 fixed at specific positions around the outside edge of the inner periphery of the annular support member 34, forming the pole piece 30.
36 in the figure is a slit that divides the soft iron magnetic ring having a trapezoidal cross-sectional shape and constituting the annular protrusion 32 into a plurality of sections in the circumferential direction. 37 is an insulating material composed of an insulating adhesive tape or the like.
By using the pole pieces 10, 20, and 30 shown in FIGS. 15(a) and (b), 17(a) and (b), and 18(a) and (b) as above, is it possible to greatly reduce the generation of residual magnetism and eddy current in the pole pieces that is caused by the Gradient magnetic field coils as compared to when the conventional pole pieces composed of a bulk magnetic material shown in FIGS. 13(a) and (b) and 14(a) and (b) are used.
However, there is a growing need for an MRI magnetic field generator capable of producing sharp images at even higher speed, and further improvement is desired.
It has been confirmed in experiments conducted by the inventors that the structures of the above-mentioned pole pieces 10 and 20 in FIGS. 15(a) and (b) and 17(a) and (b) have numerous advantages, such as producing excellent mechanical strength (rigidity) for the pole piece as a whole because of the use of the magnetic base members 11 and 12 composed of a bulk material, and affording easy assembly work because of how easy it is to laminate and lay out the plurality of laminated blocks 13 and 23 produced by laminating silicon steel sheets in a specific direction and integrating these with an insulating adhesive agent or the like. Nevertheless, the very presence of these magnetic base members 11 and 21 prevents any further reduction in the residual magnetism and eddy current in the pole pieces.
Specifically, it has been confirmed that the magnetic field generated by the Gradient magnetic field coils goes from the laminated blocks 13 and 23 directly under the Gradient magnetic field coils, through the magnetic base members 11 and 21 on which these laminated blocks 13 and 23 are placed, and reaches the surface of the soft iron magnetic ring that constitutes the annular projection 12 and 22. Therefore, the magnetic base members 11 and 21 end up being present along the magnetic path between the laminated blocks 13 and 23 and the soft iron magnetic ring, and as a result an eddy current and residual magnetism are generated within the magnetic base members 11 and 21 composed of a bulk material.
With the pole pieces 30 structured as in FIGS. 18(a) and (b), the effective use of the annular support member 34 affords the same excellent mechanical strength and ease of assembly as the structures of the pole pieces 10 and 20 in FIGS. 15(a) and (b) and 17(a) and (b).
With this structure, no magnetic base members 11 and 21 composed of a bulk material, such as those used for the pole pieces 10 and 20 in FIGS. 15(a) and (b) and 17(a) and (b), are present under the laminated rods 33 directly beneath the Gradient magnetic field coils, which is preferable from the standpoint of reducing eddy current and residual magnetism, but because the annular support member 34 present under the laminated rods 33 is also composed of a bulk magnetic material, the result is that the required reduction in eddy current and residual magnetism cannot necessarily be achieved at the present time.
Furthermore, the void 39 is formed without the inner peripheral surface of the annular support member 34 being in complete contact with the laminated rods 33c, and as a result, the ratio (Sb/Sa) between the overall surface area Sa on the side of annular protrusion 32 facing the laminated silicon steel sheets and the overall surface area Sb on the side of the laminated silicon steel sheets facing the annular protrusion 32 is less than 80% (about 70 to 75%), resulting in a magnetically unsaturated state occurring where the annular protrusion 32 meets the laminated silicon steel sheets, and this sometimes impedes the flow of the magnetic flux to the annular protrusion 32 and makes it difficult to efficiently obtain a specific uniform magnetic field within the air gap between the pole pieces.
Specifically, the flux density produced by the magnetic field from the field generation source is far higher where the annular protrusion 32 meets the laminated silicon steel sheets than in other portions, and the laminated silicon steel sheets in contact with the annular protrusion 32 in particular need to have enough volume to avoid a magnetically unsaturated state. The inventors have confirmed, however, that the uniform magnetic field originally required for an MRI magnetic field generator cannot be obtained with the structure shown in FIGS. 18(a) and (b).
It is an object of the present invention to provide an MRI magnetic field generator that solves the above problems, and it is a further object to provide an MRI magnetic field generator with which it is possible to lower the residual magnetism and eddy current within pole pieces generated by the effect of the pulse current flowing through Gradient magnetic field coils, without decreasing the field uniformity within the air gap.
The inventors perfected the invention upon learning that the stated object can be effectively achieved by optimizing the disposition of the laminate of silicon steel sheets.
Specifically, the present invention is an MRI magnetic field generator that has a pair of pole pieces facing each other so as to form a air gap and that generates a magnetic field in this air gap, wherein the pole pieces each comprise a main component consisting of laminated silicon steel sheets, and a magnetic annular protrusion disposed on the side of the main component facing the air gap.
The inventors also propose as favorable structures a structure in which the main component consisting of laminated silicon steel sheets is fixed and supported by a non-magnetic support member with high electrical resistance; a structure in which the main component is fixed and supported by a magnetic annular support member divided into a plurality of sections in the circunferential direction; a structure in which the magnetic annular protrusion consists of laminated silicon steel sheets in order to reduce the eddy current generated in this annular protrusion; a structure in which the annular protrusion is divided into a plurality of sections in the circumferential direction; a structure in which a tiered protrusion comprising laminated silicon steel sheets is formed on the inside of the magnetic annular protrusion and on the side of the pole piece main component facing the air gap; and so on.
FIG. 1 is a diagram illustrating the structure of the MRI magnetic field generator pertaining to the present invention, with (a) being a vertical cross section, (b) a top view, and (c) an oblique view of the main component;
FIG. 2 is an oblique view of the main component, illustrating the structure of the MRI magnetic field generator pertaining to the present invention;
FIG. 3 is a diagram illustrating another structure of the MRI magnetic field generator pertaining to the present invention, with (a) being a vertical cross section, (b) a top view, and (c) an oblique view of the main component;
FIG. 4 is a diagram illustrating another structure of the MRI magnetic field generator pertaining to the present invention, with (a) being a vertical cross section, (b) a top view, and (c) an oblique view of the main component;
FIG. 5 is a diagram illustrating another structure of the MRI magnetic field generator pertaining to the present invention, with (a) being a vertical cross section, (b) a top view, and (c) an oblique view of the main component;
FIG. 6 is a diagram illustrating another structure of the MRI magnetic field generator pertaining to the present invention, with (a) being a vertical cross section, (b) a top view, and (c) an oblique view of the main component;
FIGS. 7(a), (b), (c), (d), and (e) are oblique views of the structure of the magnetic annular protrusion of the MRI magnetic field generator pertaining to the present invention;
FIG. 8 is a vertical cross section illustrating a detail view of the MRI magnetic field generator pertaining to the present invention;
FIG. 9 is a vertical cross section illustrating a detail view of the MRI magnetic field generator pertaining to the present invention;
FIG. 10(a) is a vertical cross section illustrating a detail view of the MRI magnetic field generator pertaining to the present invention, and (b) is a top view;
FIGS. 11(a) and (b) are vertical cross sections illustrating detail views of the MRI magnetic field generator pertaining to the present invention;
FIG. 12 is a graph of the relationship between magnetic field uniformity and distance from the center in the air gap;
FIG. 13 is a diagram illustrating the structure of a conventional MRI magnetic field generator, with (a) being a front view and (b) a lateral cross section;
FIG. 14 is a diagram illustrating another structure of a conventional MRI magnetic field generator, with (a) being a front view and (b) a lateral cross section;
FIG. 15 is a diagram illustrating another structure of a conventional MRI magnetic field generator, with (a) being a vertical cross section and (b) a front view;
FIGS. 16(a) and (b) are oblique views of the laminated blocks used in a conventional URI magnetic field generator;
FIG. 17 is a diagram illustrating another structure of a conventional MRI magnetic field generator, with (a) being a vertical cross section, (b) a front view, and (c) an oblique view of the laminated blocks; and
FIG. 18 is a diagram illustrating another structure of a conventional MRI magnetic field generator, with (a) being a vertical cross section, (b) a front view, and (c) an oblique view of the laminated rods.